Mesoporous zeolites are of great utility in many industrial fields, both as catalysts and catalyst supports, but also as adsorbents, in so far as their large porosity expressed in terms of the [surface area/volume] ratio allows the molecules with which they come into contact readily to access the core of the particles and to react on a large surface area, thus enhancing the catalytic and/or adsorbent properties of these materials.
The synthesis of inorganic mesoporous solids via a surfactant structuring effect was described for the first time in U.S. Pat. No. 3,556,725.
The Mobil company, in the 1990s, undertook extensive studies relating to mesoporous inorganic solids, especially relating to (alumino)silicic compounds, and more particularly the compound MCM 41 (for Mobil Composition Of Matter 41), for which a synthetic process is described in Nature, (1992), Vol. 359, pp. 710-712, which were the subject of numerous subsequent scientific patents and articles.
Such mesoporous materials are now well known at the laboratory scale, both as regards their pore structure and distribution and their modes of synthesis, and as regards the possible applications thereof as catalysts and/or as adsorbents.
These mesoporous inorganic materials have the major drawback of being thermally unstable in the presence of water, which greatly limits the industrial applications.
The search for mesoporous inorganic solids led to the development of mesoporous zeolites, also known as hierarchically porous zeolites (or else HPZ). They may be obtained by various processes, such as, for example, the one described in the article by Feng-Shou Xiao et al. (Hierarchically Structured Porous Materials, (2012), 435-455, Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany 978-3-527-32788-1).
One of the routes envisaged is that of post-treatments using initially synthesized zeolite crystals, in powder form. These post-treatments are, for example, treatments with water vapour, followed by acidic and/or basic treatments which lead to dealumination, followed by additional treatments to remove extra-network species.
U.S. Pat. No. 8,486,369 and patent applications US 2013/0 183 229 and US 2013/0 183 231 are examples which illustrate such processes for preparing zeolite of mesoporous structure, in particular of Y type, by various treatments successive to vapour, and then with acids and in the presence of surfactant.
Patent WO 2012/084276 describes a process for preparing a mesoporous zeolite Y by various basic post-treatments, but to the detriment of the microporosity. These treatments furthermore result, as claimed, in an increase of the Si/Al ratio via dealumination. The zeolite Y powder thus treated is then shaped for use in a hydroconversion catalysis process.
Such processes have a tendency to create large pore volumes, but, in counterpart, greatly degrade the crystallinity of the initial zeolite powder, which is almost halved. It is moreover necessary to resort to additional cauterization treatments to stabilize the zeolite framework, to remove the extra-network aluminium atoms and thus to be able to perform subsequent heat treatments.
Such processes are therefore very cumbersome to implement due to the succession of numerous steps, which are sparingly economic and thus difficult to industrialize. In addition, the multitude of steps has a tendency to embrittle the zeolite structure and consequently reduce the intrinsic properties of these zeolites.
This is why syntheses of mesoporous zeolites directly and without post-treatment known in the prior art are nowadays preferred. Various publications show the feasibility of the laboratory synthesis of mesoporous zeolites, and, by way of example, patent applications WO 2007/043 731 and EP 2 592 049 are noted in particular, in which the synthesis of mesoporous zeolites is performed based on surfactant, and especially that of TPOAC type ([3-(trimethoxysilyl)propyl]octadecyldimethylammonium chloride).
Yet other publications illustrate such studies, for instance those of R. Ryoo (Nature Materials, (2006), Vol. 5, p. 718-723) which describe a synthesis of LTA with mesopores, or those of W. Schwieger (Angew. Chem. Int. Ed., 51, (2012), 1962-1965) which describe the synthesis of mesoporous FAU (X) using TPHAC ([3-(trimethoxysilyl)propyl]hexadecyldimethylammonium chloride) as structuring agent.
However, there is not at the present time any description concerning the preparation of agglomerates based on mesoporous Y-type FAU zeolites (or hierarchically porous zeolite Y, otherwise known as “HPY”), in which agglomerates the specific properties of these mesoporous zeolites, in particular their microporosity, are conserved. As a result, the fact remains that at the present time not one industrial application, especially in the field of separation of liquids and/or gases, of ion exchange or in the field of catalysis, uses such zeolite agglomerates with high microporosity comprising at least one mesoporous Y-type FAU zeolite (known as “HPY”).
It should be recalled that the industry, and especially the fields of application mentioned above, uses in the majority of cases zeolite agglomerates. In point of fact, synthetic zeolites are usually obtained after a process of nucleation and crystallization of silicoaluminate gels in which the size of the crystallites produced is from about a micrometer to several micrometers: they are then referred to as zeolite crystals in powder form.
These powders are not easy to use industrially since they are difficult to manipulate on account of their poor flowability, they generate substantial losses of pressure and poor distribution of the streams in the beds, especially in dynamic processes involving fluids in flow.
Thus, the agglomerated forms of these powders are preferred, which are more commonly referred to as zeolite agglomerates and which may be in the form of grains, strands, extrudates or other agglomerates, these said forms possibly being obtained by extrusion, pelletizing, atomization or other agglomeration techniques that are well known to those skilled in the art. These agglomerates do not have the drawbacks inherent in pulverulant materials.
These agglomerates generally consist of zeolite crystals and of a binder, which is usually inert with respect to the application for which the zeolite is intended, the said binder being intended to ensure the cohesion of the zeolite crystals and to give them the sufficient and necessary mechanical strength for the intended industrial application.